Re: linux-4.4 bisected: kwin5 stuck on kde5 loading screen with radeon

[Mario Kleiner <mario.kleiner.de@gmail.com>]

On 01/25/2016 05:15 AM, Michel Dänzer wrote:
> On 23.01.2016 00:18, Ville Syrjälä wrote:
>> On Fri, Jan 22, 2016 at 12:06:00PM +0900, Michel Dänzer wrote:
>>>
>>> [ Trimming KDE folks from Cc ]
>>>
>>> On 21.01.2016 19:09, Daniel Vetter wrote:
>>>> On Thu, Jan 21, 2016 at 05:36:46PM +0900, Michel Dänzer wrote:
>>>>> On 21.01.2016 16:58, Daniel Vetter wrote:
>>>>>>
>>>>>> Can you please point me at the vblank on/off jump bug please?
>>>>>
>>>>> AFAIR I originally reported it in response to
>>>>> http://lists.freedesktop.org/archives/dri-devel/2015-August/087841.html
>>>>> , but I can't find that in the archives, so maybe that was just on IRC.
>>>>> See
>>>>> http://lists.freedesktop.org/archives/dri-devel/2016-January/099122.html
>>>>> . Basically, I ran into the bug fixed by your patch because the counter
>>>>> jumped forward on every DPMS off, so it hit the 32-bit boundary after
>>>>> just a few days.
>>>>
>>>> Ok, so just uncovered the overflow bug.
>>>
>>> Not sure what you mean by "just", but to be clear: The drm_vblank_on/off
>>> counter jumping bug (similar to the bug this thread is about), which
>>> exposed the overflow bug, is still alive and kicking in 4.5. It seems
>>> to happen when turning off the CRTC:
>>>
>>> [drm:drm_update_vblank_count] updating vblank count on crtc 0: current=218104694, diff=0, hw=916 hw_last=916
>>> [drm:radeon_get_vblank_counter_kms] crtc 0: dist from vblank start 3
>>> [drm:drm_calc_vbltimestamp_from_scanoutpos] crtc 0 : v 0x7 p(2199,-45)@ 7304.307354 -> 7304.308006 [e 0 us, 0 rep]
>>> [drm:radeon_get_vblank_counter_kms] crtc 0: dist from vblank start 3
>>> [drm:drm_update_vblank_count] updating vblank count on crtc 0: current=218104694, diff=16776301, hw=1 hw_last=916
>>
>> Not sure what bug we're talking about here, but here the hw counter
>> clearly jumps backwards.
>>
>>> [drm:radeon_get_vblank_counter_kms] Query failed! stat 3
>>> [drm:radeon_get_vblank_counter_kms] Query failed! stat 3
>>> [drm:drm_update_vblank_count] updating vblank count on crtc 1: current=0, diff=0, hw=0 hw_last=0
>>> [drm:radeon_get_vblank_counter_kms] Query failed! stat 3
>>> [drm:radeon_get_vblank_counter_kms] Query failed! stat 3
>>> [drm:drm_update_vblank_count] updating vblank count on crtc 2: current=0, diff=0, hw=0 hw_last=0
>>> [drm:radeon_get_vblank_counter_kms] Query failed! stat 3
>>> [drm:radeon_get_vblank_counter_kms] Query failed! stat 3
>>> [drm:drm_update_vblank_count] updating vblank count on crtc 3: current=0, diff=0, hw=0 hw_last=0
>>> [drm:radeon_get_vblank_counter_kms] Query failed! stat 1
>>> [drm:drm_calc_vbltimestamp_from_scanoutpos] crtc 0 : v 0x1 p(0,0)@ 7304.317140 -> 7304.317140 [e 0 us, 0 rep]
>>> [drm:radeon_get_vblank_counter_kms] Query failed! stat 1
>>> [drm:drm_update_vblank_count] updating vblank count on crtc 0: current=234880995, diff=16777215, hw=0 hw_last=1
>>
>> Same here.
>
> At least one of the jumps is expected, because this is around turning
> off the CRTC for DPMS off. Don't know yet why there are two jumps back
> though.
>
>
>> These things just don't happen on i915 because drm_vblank_off() and
>> drm_vblank_on() are always called around the times when the hw counter
>> might get reset. Or at least that's how it should be.
>
> Which is of course the idea of Daniel's patch (which is what I'm getting
> the above with) or Mario's patch as well, but clearly something's still
> wrong. It's certainly possible that it's something in the driver, but
> since calling drm_vblank_pre/post_modeset from the same places seems to
> work fine (ignoring the regression discussed in this thread)... Do
> drm_vblank_on/off require something else to handle this correctly?
>
>

I suspect it is because vblank_disable_and_save calls 
drm_update_vblank_count() unconditionally, even if vblank irqs are 
already off.

So on a manual display disable -> reenable you get something like

At disable:

Call to dpms-off --> atombios_crtc_dpms(DPMS_OFF) --> drm_vblank_off -> 
vblank_disable_and_save -> irqs off, drm_update_vblank_count() computes 
final count.

Then the crtc is shut down and its hw counter resets to zero.

At reenable:

Modesetting -> drm_crtc_helper_set_mode -> crtc_funcs->prepare(crtc) -> 
atombios_crtc_prepare() -> atombios_crtc_dpms(DPMS_OFF) -> 
drm_vblank_off -> vblank_disable_and_save -> A pointless 
drm_update_vblank_count() while the hw counter is already reset to zero 
--> Unwanted counter jump.


The problem doesn't happen on a pure modeset to a different video 
resolution/refresh rate, as then we only have one call into 
atombios_crtc_dpms(DPMS_OFF).

I think the fix is to fix vblank_disable_and_save() to only call 
drm_update_vblank_count() if vblank irqs get actually disabled, not on 
no-op calls. I will try that now.

Otherwise kms drivers would have to be careful to never call 
drm_vblank_off multiple times before calling drm_vblank_on, but the help 
text to drm_vblank_on() claims that unbalanced calls to these functions 
are perfectly fine.

-mario